The present invention relates to a large-scale hydrogen arc discharge method for the synthesis of single-walled carbon nanotubes (xe2x80x9cSWNTsxe2x80x9d). Under an atmosphere of H2, a large-size anode composed of graphite powders, catalyst metals and growth promoter is consumed with the growth of SWNTs. The angle between the anode and cathode can be adjusted in the range of 30xc2x0xcx9c80xc2x0. The catalysts used can be two or more kinds of metals selected from Fe, Co, Ni and Y, which have proven to be essential for the growth of SWNTs. The proportion of catalysts is modified in the range of 2.5xcx9c5.0 at %. By these processes, large-scale SWNTs with a large diameter can be synthesized with the advantages of semi-continuous synthesis, low cost and high quality, and the SWNTs obtained can then be employed as a kind of promising hydrogen storage material.
To eliminate the gasoline shortage and environmental pollution produced by automobiles, various technologies are being developed to replace the gasoline-powered internal combustion engine. Among the alternative fuel technologies, the hydrogen fuel cell is believed to be ideal because of its advantages of renewable and non-pollution performance. For the purpose of identifying the practicability of the fuel cell powered vehicle, the United States Department of Energy (xe2x80x9cD.O.E.xe2x80x9d) has issued energy density goals for vehicular hydrogen storage: 6.5 wt % H2 or 62 kg H2/m3, which indicates that the identified fuel cell can provide a 350 mile range driving in a vehicle.
Compared to all the other hydrogen storage systems, such as liquid hydrogen systems, compressed hydrogen systems, metal hydride systems and super active carbon systems, carbon nanotube systems, especially single-walled carbon nanotubes (SWNTs) are expected to be utmost close to the D.O.E. energy density goals and can be made into an ideal hydrogen storage system, which needs to be light, compact, relatively inexpensive, safe, easy to use and reusable without the need for regeneration.
The most widely used method to produce SWNTs is the electric arc discharge method, which was first employed by S. Iijima and Bethune in 1993. However, the purity of the products they obtained was very poor, so that the characterization and utilization of SWNTs were limited to a great extent. In 1997, C. Journet et al obtained SWNTs with higher yield and purity through the optimization of synthesis parameters. The main characteristics of their method include: use of He at 660 torr as a buffer gas, use of two graphite rods as electrodes, and the size of the electrodes were: anodexe2x80x94"PHgr" 16xc3x9740 mm, cathodexe2x80x94"PHgr" 16xc3x97100 mm, and a "PHgr" 3.5xc3x9740 mm pore was drilled along with the axis of the anode to fill with a mixture of graphite powders and catalysts (Y, Ni). The anode and the cathode were perpendicular with each other. The morphologies of the products can be classified as follows: (1) Rubbery soot formed on the internal wall of the reactor; (2) Web-like substance between the cathode and reactor wall; (3) Cylinder deposit on the tip of the cathode; and (4) Porous, light, hoop-like substance formed around the cylinder deposit. From then on, no remarkable progress happened with respect to the electric arc discharge method. Although the obtained SWNTs have nearly defect-free microstructure, the traditional electric arc discharge method is difficult to apply to commercial or large-scale synthesis because of its shortcomings: i.e., the yield is limited by the size of electrodes and the reactor; the producing process is un-continuous; and the content of impurities such as amorphous carbon is high.
Since SWNTs have aroused a great deal of interest from the fundamental viewpoint as well as for potential applications, enormous high-quality SWNTs are demanded for both research and commercial purposes. Consequently, there is an urgent need for a new method which can produce SWNTs in a semi-continuous or continuous synthesis in large-scale, high quality and low cost.
In accordance with the present invention, there is a hydrogen arc discharge method provided for the production of SWNTs with high hydrogen storage capacity. The main characteristics or parameters of the method include:
(a) Large-scale SWNTs are produced with: (i) graphite powders, (ii) catalyst metals selected from Fe, Co, Ni, Y, (iii) sulfur or solid sulfide as a growth promoter, and (iv) H2 at a pressure of 50xcx9c400 torr as a buffer gas. After being evenly dispersed, the reactants are filled into holes provided on the upper surface of a large anode or mold-pressed into targets as the anode, and the anode will be consumed with the growth of SWNTs under the atmosphere of H2; and
(b) The obtained SWNTs are (i) soaked in acids or oxidative reactants, (ii) heated in vacuum, or (iii) through a combined pre-treatment of (i) and (ii), to prepare high-capacity hydrogen storage materials.
In preferred embodiments of the present invention, the diameter of the anode is about 10xcx9c20 times larger than that of the cathode, so that the synthesis time is enlonged to produce large-scale SWNTs.
In other preferred embodiments of the present invention, the angle between the anode and the cathode can be adjusted from 30xc2x0 to 80xc2x0. By changing the angle, the quality and morphology of the products can be changed.
In still other preferred embodiments of the present invention, argon (no more than 20 vol %) can be mixed into the H2 as a buffer gas.
In yet other preferred embodiments of the present invention, the pre-treatment techniques for the SWNTs may include: (a) soaking in (i) HNO3 acid (20xcx9c65%), (ii) HCl acid (10xcx9c37%) or (iii) oxidative reactants, (b) vacuum heating at 400xcx9c900xc2x0 C. or (c) the combination of (a) and (b).